A speed ratio of an automobile is governed by a mechanism for transmitting torque from a prime mover such as an engine to drive wheels. In general, a gear stage of a vehicle transmission including a forward stage and a reverse stage is selected by selecting a gear pair for transmitting the torque from a plurality of gear pairs by a clutch. Given that the transmission is formed by combining a plurality of planetary gear units, a torque transmission route is changed depending on engagement states of a clutch and a brake. Alternatively, the gear stage of the transmission may also be shifted by changing engagement states of the clutch and the brake thereby changing a reaction element or an output element of the planetary gear unit. In the transmissions of those kinds, the gear stage including the forward stage and the revers stage can be changed by disengaging the engagement device engaged to establish the current gear stage while engaging another engagement device to establish another gear stage. Such method is called “the clutch-to-clutch shifting”. However, if torque transmitting capacities of both engagement devices to be engaged and disengaged are insufficient during the clutch-to-clutch shifting, so-called a “retraction” feeling may be caused by a torque drop. By contrast, if torque transmitting capacities of both engagement devices to be engaged and disengaged are excessive, shocks may be caused by a torque reduction resulting from a transmission tie-up.
Therefore, the torque transmitting capacities of the engagement devices to be engaged and disengaged have to be controlled in just proportion. Various kinds of control systems and methods have been proposed for this application. For example, Japanese Patent Laid-Open No. 2008-051186 describes a shift control system. According to the teachings of Japanese Patent Laid-Open No. 2008-051186, a torque of one of clutches involved in the clutch-to-clutch shifting is determined based on a torque sharing rate between a target output torque of a transmission and a torque of said one of the clutches, and a torque of the other clutch is determined based on the torque of said one of the clutches and the target output torque. In addition, the torque of the clutch to be disengaged is reduced gradually in a manner such that the torque becomes zero at a commencement of an inertia phase. Accordingly, the torque will not be applied to the clutch to be disengaged during the inertia phase, and the shifting operation is progressed by increasing hydraulic pressure applied to the clutch to be engaged. In addition, according to the teachings of Japanese Patent Laid-Open No. 2008-051186, the torque sharing rate is differentiated before and after the commencement of the inertia phase. Specifically, the torque sharing rate of the clutch for the former gear stage is employed before the commencement of the inertia phase, and the torque sharing rate of the clutch for the gear stage to be achieved after shifting is employed after the commencement of the inertia phase.
Japanese Patent Laid-Open No. 2001-227637 also describes a control device for automatic transmission configured to change a torque transmitting capacity of the clutch to be engaged in accordance with a change in an input speed during a torque phase of the clutch-to-clutch shifting. If the torque transmitting capacity of the clutch to be engaged is insufficient during the clutch-to-clutch shifting, an engine speed or an input speed is increased while the power is on. By contrast, if the torque transmitting capacity of the clutch to be engaged is excessive, the “retraction” feeling may be caused by a torque drop resulting from the transmission tie-up. In order to avoid such disadvantages, according to the teachings of Japanese Patent Laid-Open No. 2001-227637, the torque transmitting capacity of the clutch to be engaged is increased in case the input speed is increased, and the torque transmitting capacity of the clutch to be engaged is decreased in case the input speed is lowered.
Japanese Patent Laid-Open No. 2008-025637 describes a control apparatus and method for automatic transmission. According to the teachings of Japanese Patent Laid-Open No. 2008-025637, during the clutch-to-clutch shifting, an input speed and an output speed of a clutch to be disengaged are differentiated by disengaging the clutch to be disengaged. In order to achieve a target speed difference therebetween, a total torque capacity of first and second clutch is calculated, and the torque capacity is distributed to the first and the second clutches in accordance with a determined distribution ratio.
As described, according to the teachings of Japanese Patent Laid-Open No. 2008-051186, the torque transmitting capacity of the clutch to be disengaged or the hydraulic pressure applied thereto is reduced to zero at the commencement of the inertia phase. Therefore, if the torque transmitting capacity of the clutch to be disengaged or the hydraulic pressure applied thereto is increased to a certain extent just before the commencement of the inertia phase, the clutch is disengaged abruptly at the commencement of the inertia phase. In this situation, the torque transmitting capacity of the clutch to be engaged or the hydraulic pressure applied thereto has to be increased abruptly to make up for the reduction in the torque transmitting capacity of the disengaged clutch. Thus, the torque transmitting capacity of the clutch or the hydraulic pressure applied thereto has to be controlled irregularly thereby causing shift shocks. In addition, in order to reduce the torque transmitting capacity of the clutch to be disengaged to zero at the commencement of the inertia phase, man-hour at design phase may be increased. Further, if the torque transmitting capacity of the clutch to be disengaged is thus determined based on the target output torque, the torque transmitting capacity of the clutch to be disengaged has to be changed in response to a change in an accelerator opening or the target output torque during shifting. Consequently, shift shocks may be worsened due to control delay. In addition, a control system and a control software has to be complicated to deal with such disadvantages.
As also described, the control device taught by Japanese Patent Laid-Open No. 2001-227637 is configured to control the clutch to be engaged during the torque phase of the clutch-to-clutch shifting. According to the teachings of Japanese Patent Laid-Open No. 2001-227637, rotational speeds of rotary members including the engine are changed positively. Therefore, the teachings of Japanese Patent Laid-Open No. 2001-227637 may not be applied to control entire course of the clutch-to-clutch shifting.
The control apparatus and method taught by Japanese Patent Laid-Open No. 2008-025637 are configured to simplify the control of downshifting. Therefore, the control apparatus and method of Japanese Patent Laid-Open No. 2008-025637 cannot be applied to all of the shifting modes of the clutch-to-clutch shifting including upshifting. In addition, according to the teachings of Japanese Patent Laid-Open No. 2008-025637, the shifting operation is carried out while causing a slip of the clutch to be disengaged. Therefore, the clutch may be worn out and damaged.